Mechanical ribcage having variable number of ribs and derived heights
Method 1: even rib size

You can use a combination of some modifiers such as:


Array (using object offset pointing to an empty to get the step
distance to increase; notice that the empty doesn't have the same Y coordinate of the object origin, so each step the repeated object gets nearer to the center)
Boolean (to cut away the parts that exceed the middle line with a "control" box)
Mirror (to repeat the other half)




Method 2: slightly deformed ribs

Use a regular X-axis Array modifier (much simpler to set up than the array previusly presented) and deform the result with a lattice with two control points only in order for each direction (U,V,W) to get a linear deformation:



While the second method is probably faster, the first would give you more control and precision.